It can push fluid past the cap if the fluid is foamy. I had symptoms similar to yours. I had a leaky pump shaft seal and was losing lots of fluid through the vacuum pump housing and into the timing case. I think I remember having leakage around the cap even when the fluid level was low. Air entrainment due to low oil level may cause the foam and possibly increase the rate of leakage. I'd top off the reservoir and drive it and it would work better for a while, but the problem would return. I eventually bought an inexpensive repair kit for the pump from Autozone and my problem was cured. This may be what you are experiencing. I think the low fluid level and corresponding air entrainment and air circulation throughout the system can create a vicious circle under the right conditions. I wrote a post about working on the pump several months back.